In integrated circuits, such as microprocessors, memories, and the like, signals may be routed for relatively long distances using transmission lines. A transmission line may be a bus, a printed circuit board trace, or other type of relatively long metal line for transporting a digital signal. Typically, a printed circuit board trace has a characteristic impedance of between 50 and 75 ohms. The receiving end, or far end, of the transmission line is typically connected to an input of a logic circuit, where the input impedance is higher than the characteristic impedance of the transmission line. If the impedance coupled to the far end of the transmission line is different than the impedance of the transmission line, the signal may be reflected back to the sending end, causing the signal to overshoot a planned steady state voltage for the logic state. The signal may be reflected back and forth many times between the near end and the far end, causing oscillatory behavior of the signal at both ends. This repeated overshooting and undershooting of the signal is commonly known as "ringing", and results in reduced noise immunity and increased time for the signal to become, and remain, valid at the far end.
A driver circuit is used to provide enough current to "drive" a signal the length of the transmission line. A CMOS (complementary metal-oxide semiconductor) driver circuit commonly includes a P-channel transistor and an N-channel transistor connected in series between a positive power supply voltage terminal and a ground terminal. The gates of the transistors receive an input signal, and an output terminal of the driver circuit is located between the transistors. The P-channel transistor functions as a "pull-up" transistor, and the N-channel transistor functions as a "pull-down" transistor. The output impedance of the driver circuit should match the characteristic impedance of the transmission line in order for the driver circuit to absorb the reflected signal and prevent ringing. This would result in quieter waveforms, better noise immunity, and improved signal timing margins. However, the load impedance that a driver circuit is required to drive may vary, depending on the particular application in which the driver circuit is installed.
GTL (Gunning Transceiver Logic) is a logic type that is becoming increasingly popular. GTL has a logic swing that is comparable to the well known ECL (emitter-coupled logic) logic type. However, GTL has relatively low power consumption, high speed, and can be implemented with a CMOS process. In integrated circuits using logic levels having relatively small logic swings, such as GTL levels, even a small amount of ringing can result in significantly reduced noise immunity. Also, the signal timing margins as compared to, for example CMOS logic levels, may be significantly impaired.